Can I use a LM324N (very common) op-amp to act as a high-side differential current sense op amp at high voltage?
The Differential Amplifier
The differential input voltage rating is 32v, but my load supply is up to 80v. I have found more expensive current sense ICs that have input protection up to 500v, but I like the 324 because it's super cheap and comes in quad packages.
The actual voltage across the amplifier's inputs will not exceed very small values during sensing, and technically the two terminals of the 1mOhm sense resistor should never have more than 32v differential potential between them, but when idle, the high side resistor may have a difference of 80ish volts between the negative supply (GND) and the inputs.
Will this blow out the opamp? If so, can I use some clever diode protection trick to keep the opamp input voltage down?
Best Answer
Yes, it will probably work*, and if the resistors R1 and R2 have high enough value to limit current to a safe value, the opamp will be protected.
However the common mode rejection of this type of difference amplifier depends on accurate matching between resistors. If you want decent CMRR it is quite likely that will require precision resistors which will end up more expensive than a ready-made current sense amp.
Also... * you didn't specify what gain and offset you want. So by "work*" I mean it will probably output a signal, but it will probably not be the signal you want.
I recommend doing the math on the circuit in your question, taking into account resistor tolerance and opamp input offset voltage and current. Is the gain you want possible with this configuration without clipping the opamp? This should give you a better understanding of the issues.